gdb: don't include hashtab.h in defs.h

Nothing in defs.h actually uses this.

Add some includes for some spots using things from hashtab.h.  Note that
if the GDB build doesn't use libxxhash, hashtab.h is included by
gdbsupport/common-utils.h, so all files still see hashtab.h.  It puzzled
me for some time why I didn't see build failures in my build (which
didn't use libxxhash) but the buildbot gave build failures (it uses
libxxhash).

Change-Id: I8efd68decdaf579f048941c7537cd689885caa2a
Approved-By: John Baldwin <jhb@FreeBSD.org>

gdb/defs.h

@@ -82,8 +82,6 @@
 #define O_BINARY 0
 #endif
 
-#include "hashtab.h"
-
 /* * System root path, used to find libraries etc.  */
 extern std::string gdb_sysroot;
 

gdb/dwarf2/die.h

@@ -22,6 +22,7 @@
 
 #include "complaints.h"
 #include "dwarf2/attribute.h"
+#include "hashtab.h"
 
 /* This data structure holds a complete die structure.  */
 struct die_info

gdb/dwarf2/index-common.h

@@ -21,6 +21,7 @@
 #define DWARF_INDEX_COMMON_H
 
 #include "extract-store-integer.h"
+#include "hashtab.h"
 
 /* The suffix for an index file.  */
 #define INDEX4_SUFFIX ".gdb-index"

gdb/extension.h

@@ -22,6 +22,7 @@
 
 #include "mi/mi-cmds.h"
 #include "gdbsupport/array-view.h"
+#include "hashtab.h"
 #include <optional>
 
 struct breakpoint;